Additive Manufacturing

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Additive Manufacturing

journal homepage:www.elsevier.com/locate/addma

An investigation into 3D printing of ﬁ bre reinforced thermoplastic composites

L.G. Blok

^⁎

, M.L. Longana, H. Yu, B.K.S. Woods

Bristol Composites Institute (ACCIS), University of Bristol, Bristol, BS8 1TR, UK

A R T I C L E I N F O

Keywords:

Composite Thermoplastic 3D printing Additive manufacture

A B S T R A C T

Fusedfilament fabrication (FFF) is a 3D printing technique which allows layer-by-layer build-up of a part by the deposition of thermoplastic material through a nozzle. The technique allows for complex shapes to be made with a degree of design freedom unachievable with traditional manufacturing methods. However, the mechanical properties of the thermoplastic materials used are low compared to common engineering materials. In this work, composite 3D printing feedstocks for FFF are investigated, wherein carbonfibres are embedded into a thermoplastic matrix to increase strength and stiffness. First, the key processing parameters for FFF are reviewed, showing howfibres alter the printing dynamics by changing the viscosity and the thermal profile of the printed material. The state-of-the-art in composite 3D printing is presented, showing a distinction between shortfibre feedstocks versus continuousfibre feedstocks. An experimental study was performed to benchmark these two methods. It is found that printing of continuous carbonfibres using the MarkOne printer gives significant increases in performance over unreinforced thermoplastics, with mechanical properties in the same order of magnitude of typical unidirectional epoxy matrix composites. The method, however, is limited in design freedom as the brittle continuous carbonfibres cannot be deposited freely through small steering radii and sharp angles.

Filaments with embedded short carbon microfibres (∼100μm) show better print capabilities and are suitable for use with standard printing methods, but only offer a slight increase in mechanical properties over the pure thermoplastic properties. It is hypothesized that increasing thefibre length in shortfibrefilament is expected to lead to increased mechanical properties, potentially approaching those of continuousfibre composites, whilst keeping the high degree of design freedom of the FFF process.

1. Introduction

Carbon fibre reinforced plastics (CFRPs) provide excellent mechanical properties and allow for significant design tailorability. A fundamental CFRP manufacturing challenge, however, is the combi- nation of the reinforcementfibres into the polymer matrix with good consolidation, control of fibre orientation and low cost [1]. While a wide range of manufacturing methods for composites are available, most CFRP parts are formed in a two-stage process, i.e. material lay-up followed by consolidation. For the second stage, pressure needs to be applied over the entire part surface area which requires expensive equipment and increases manufacturing costs. In this work, fusedfilament fabrication (FFF) is investigated as an alternative CFRP manufacturing approach for low to medium production volumes and highly customizable parts, e.g. rapid prototyping, personalised devices or structures with complex geometry.

Additive manufacturing techniques, such as FFF, commonly known as 3D printing, have an underappreciated similarity to those of

traditional composite materials, as both are inherently based on stacking a series of discrete layers. It is therefore reasonable to suggest that successful adaptation of 3D printing technologies to composite materials could enable a simple composite manufacturing method with lower production cost and a high degree of automation. As reinforcements can be accurately placed, the laminated structure of composite parts can be further optimised in each layer, allowing for an increase in design freedom and mechanical performance. While still a relatively undeveloped avenue of research, there is at least one company devel- oping commercial 3D printers capable of processing continuousﬁbre reinforced composite materials: MarkForged [2]. The Mark One and Mark Two printers developed by MarkForged print continuous carbon ﬁbre reinforced Nylon with mechanical properties an order of magnitude higher than common 3D printers, and open new applications in both the personal fabrication market and in the manufacture of light- weight parts for industry.

Signiﬁcant challenges remain for 3D printing of CFRPs. In addition to some process speciﬁc limitations with the MarkForged printers,

https://doi.org/10.1016/j.addma.2018.04.039

Received 30 November 2017; Received in revised form 8 March 2018; Accepted 30 April 2018

⁎Corresponding author.

E-mail address:lourens.blok@bristol.ac.uk(L.G. Blok).

Available online 08 May 2018

T

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which will be discussed in further detail below, there are more fundamental issues which need addressing. For example, there are currently only a few different materials available forfibre reinforced 3D printing, which limits application areas and designflexibility. The addition of (short)fibres to the printingfilament increases the stiffness of the part but the strength increase is still limited as fibre pull out may occur before fibre breakage. Furthermore, current printing techniques and material options lead to the creation of significant voids in thefinished parts, which have a negative impact on the obtainable strength of composites [3].

In this paper, a review is presented on the body of knowledge of 3D printing of fibre composites using the FFF technique, followed by a detailed consideration of the processing parameters which dictate the final part quality. The aim is to identify to what extent FFF may be used as a composite manufacturing method, considering along the way what progress has been made and what challenges remain. Two different methods of composite 3D printing were assessed (continuous fibre printing and shortfibre printing) and comparisons were made between the two methods in terms of mechanical properties, part quality and printing versatility.

2. Review

2.1. Material extrusion processes

Material extrusion based 3D printing techniques, such as FFF and Fused Deposition Modelling (FDM), are manufacturing processes where a solid thermoplastic material is extruded through a hot nozzle. The viscous material solidiﬁes on the build plate which allows build-up of a part with dimensional accuracies typically in the order of 100μm [4].

The most commonly used thermoplastics for this process are acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA), with typical bulk strengths between 30–100 MPa and elastic moduli in the range of 1.3–3.6 GPa [5]. Mechanical properties of 3D printed parts, however, can deviate signiﬁcantly from the material bulk properties due to the speciﬁcs of how a structure is formed on the meso-scale during printing [6].

To maximise the mechanical performance of printed parts, the key elements of the printing process and how they affectfinal print quality must be understood (Fig. 1). Turner et al. [4,7] provide an extensive review on FFF process modelling, including theflow and thermal dynamics of the melt, the extrusion process and the bonding process between successive layers of material. Temperature, viscosity and surface energy of the melt play an important role in how the materialflows through the nozzle and more importantly, how thefinal interface between the beads is formed.

One of the major process variables is the raster angle, as illustrated in Fig. 2, which leads to diﬀerent properties across the principal

material directions [8–10], similar to the orthotropic behaviour ofﬁbre composites. This allows for design tailoring, but stiﬀness can be 11%

lower and tensile strength up to 50% lower in the weaker 1- and 3- directions compared to the bulk properties, as the interface between printed tracks can be weak [11,12].

Important features on the mesoscale are the contact area between the printed tracks and the minimization of the overall void content, as they can have a large eﬀect on the printed part strength. Diﬀerent printing patterns can be used to increase area of contact between the printed tracks and minimize the void content as shown inFig. 3. Several studies analysed the void density in 3D printed parts, both analytically and experimentally, with changing the gap size between tracks [6,13].

A small overlap between the tracks gave the best results, with a void density of∼5% in the 1–3 plane and 27% in the 2–3 plane. Micro- graphs taken of 3D printed structures typically show a clear mesostructure with diamond or triangular shaped interbead voids, as shown inFig. 3.

On a molecular level, good chemical bonding between the polymer chains inside of adjacent beads is required for effective load transfer to obtain a high strength part [3,14]. The amount of initial surface contact and the distribution of heat between two adjacent beads leads to the formation of a neck (Fig. 4) as absorptive equilibrium is reached (a lower state of overall energy by minimizing surface area). This process is inhibited by the viscosity of the material. During neck formation, diffusion of the polymer chains occurs while the viscosity of the material increases as it cools down, slowing down the neck formation and diffusion process [7]. This process is therefore sensitive to the viscosity (temperature dependent), thermal conductivity and heat capacity of the material, as well as the cooling rate (determined by external environ- ment). A higher temperature leads to betterflow of the polymer melt, improving the polymer sintering process. Similarly, a higher thermal conductivity would improve heat distribution, aiding the chemical bonding betweenfilaments as previously deposited material heats up to improve the sintering process. At too high temperatures, however, the polymer may degrade, and dimensional accuracy may decrease because of the increasedflow.

Multiple attempts have been made to numerically model the polymer sintering process based on heat transfer calculations. Early work by Yardimci et al. [14,15] presented diﬀerent modelling approaches to capture the heat transfer between printed beads, but did not look at the polymerﬂow dynamics. Bellehumeur et al. [16] used a model based on a polymer sintering model described by Pokluda et al.

[17]. This approach performed an energy balance between surface tension and viscous dissipation [17], but with the extension of temperature dependent surface tension and viscosity. Although they did not model molecular diffusion, they found that the extruded material cools too quickly for complete bonding. They also report that the convective heat transfer coefficient has a large effect on the bond formation and neck growth, where less heat transfer leads to better neck formation.

However, they modelled isothermal polymer sintering and did not consider the heat transfer from the hot extruded material to the surrounding material. Bellini [18] performed extensive modelling of the entire FDM process with ceramic filledfilament using four different Fig. 1.Key elements of the FDM process.

Adapted from [7].

Fig. 2.Example of meso-structure of 3D printed parts with raster angle.

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numerical simulations focusing on; the liquefier, the nozzle contraction, deposition on the printing bed and on stacked layers. This enabled tracking of the material temperature, swelling andfilling as a function of various printing parameters. It was found that the higher thermal conductivity of thefilled material increased heat transfer from the liquefier to the printed material and improved theflow behaviour.

To conclude,Fig. 5summarises the discussed printing and material parameters that influence the print quality, mapped to the different stages of the printing process. Overall, the key to high quality parts is to obtain good surface contact and temperature conditions for optimal polymer sintering. The viscosity and surface tension of the material are important parameters, as they dictate theflow characteristics which are mainly dependent on temperature. Therefore, the heat conductivity and capacity are important, as they affect how heat is distributed and the temperature profiles of the printed tracks. Qualitatively, the main sintering process is understood and several studies focused on the effect of some of these parameters [19–21]. Of further interest is how the

addition ofﬁbre reinforcement to the feedstock will aﬀect these parameters, this is discussed below.

2.2. Reinforcedﬁlaments for material extrusion

The FFF process can be utilized to print CFRPs by addingfibres into the thermoplastic filament. Besides the obvious motivation of increasing mechanical properties, the reinforcement may also be used to add extra functionality to the material such as electroconductivity, higher heat conductivity or biocompatibility. Kalsoom et al. [22] and Wang et al. [23] recently provided a general overview of 3D printable composite materials; this paper instead provides a more detailed focus on the engineering aspects of FFF as a composite manufacturing method. The use offibre reinforcements in 3D printingfilaments for FFF is a topic of on-going research with both advancements in scientific literature as well as in commercial products, e.g. the MarkForged printers and the numerous reinforced thermoplasticfilaments available on the market [2,24,31].

Table 1shows an overview of the different studies performed to date on printing of reinforcedfilaments, showing the different meth- odologies and resulting relevant mechanical properties. Most studies report on the use of very short carbonfibres (∼0.1 mm) which are mixed with a thermoplastic polymer and then typically screw extruded to create thefilament used for traditional printing. This increases the strength and stiffness of the printed material by around 65%, but this level of performance remains low compared to CFRP materials made with traditional composite manufacturing methods (e.g. pre-preg/autoclave, resin infusion, etc). High shear mixing leads tofibre breakage, reducing their length in the filament and consequently lowering the strength of the printed part [32,33].

The porosity of 3D printed shortfibre composite parts has also been investigated. Three types of voids are identified by Ning et al. [24] as shown inFig. 6. They found that the overall porosity initially decreased with the addition offibres, but atfibre contents above 10 wt% the porosity increased to almost 10% but without distinguishing between Fig. 3.Micrographs and schematics of two different meso-structures, a) rectangular and b) skewed, showing typical triangular void formation [6].

Fig. 4.Schematic overview of the polymer sintering process [7].

Fig. 5.Main parameters for good surface contact and temperature conditions to enable optimal polymer sintering conditions.

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Table1 Overviewofstudiesonprintingofreinforcedfilaments. StudyMatrixReinforcementAmountof reinforcementManufacturingtechniqueResultOther Ningetal.[35]ABSCarbonfibrepowder (L=100μm,150μmand Φ=7.2μm) 3-15wt%Mixinginblender,followedbydouble extrusionStrengthfrom34MPato42MPa, stiffnessfrom2GPato2.5GPa,decrease intoughness,yieldstrengthand ductility Anincreaseinvoidcontentincreasefrom3%to9%was recordedfor10wt%specimens Tekinalpetal. [25]ABSShortcarbonfibres (L=3.2mm,after0.26mm mixing)

10,20,30,40wt%Mixingwithtorquerheometer,followed byplungerextrusionStrengthfrom35MPato65MPa. Stiffnessfrom2GPato14GPaFor40wt%somenozzleclogging.Goodfibreorientation forprintedparts.Voidcontent16-27%. Matsuzakietal. [26]PLAContinuouscarbonfibres andjutefibresVfof6.5%Pre-heatingfibresandaddingitto thermoplasticfilamentStrengthfrom40MPato185MPa, modulusfrom4GPato20GPa,witha decreaseinmaximumstrain.

Fibrespoorlydistributedattheoutsideofthefilament duetomanufacturingtechnique,voidsreportedbutnot quantified Shofneretal. [27]ABSVaporgrowncarbon nanofibersL=100μmand Φ=0.1μm)

10wt%Sizingaddedtonanoﬁbers,banbury mixing,compressionmoulding, granulation,screwextrusion

Strengthfrom26.9MPato37.4MPaand stiffnessfrom0.49GPato0.79GPa.PooradhesionbetweenfibresandresinfoundbySEM pictures.Goodalignment. Mahajanand Cormier[28]EpoxyresinShortcarbonfibres (L=100μm,Φ=7.2μm)15wt%Mixingofepoxyresinandfibres,and printingviasyringesStrengthfrom46MPato65MPaand modulusfrom2.8GPato4.05GPa.FFTanalysiswasusedtoobtainthefibreorientation.It wasfoundthroughdesignofexperimentsthatfibre content,translationspeedandnozzlediameterhada significanteffect,whilefibrelengthandprintingpressure werelessimportant. Pengetal.[29]EpoxyresinShortglassfibres (L=0.8mm,Φ=10μm)8wt%Mixingofepoxyresinandfibresand printingviasyringesFlexuralmodulusincreasefrom4.2to 6.3GPaandflexuralmodulusfrom 91MPato109MPafromunalignedto alignedfibres.

SimilartoMahajanandCormier[28],writingspeedwas foundtohaveasignificantimpactonfibreorientation. Yangetal.[36]ABSContinuouscarbonfibre10wt%In-situimpregnationofcontinuousfibre throughmeltpoolofmatrixbefore printing

Flexuralstrengthof7127MPaand ﬂexuralmodulusof7.72GPaVerylowinterlaminarshearstrengthof2.81MPa. Lewickietal. [31]Epoxyresin, modiﬁedforfast curing

Carbonfibres(L=300and 600μmandΦ=6μm)Vfof8%DirectInkWriting,mixingofresinwith reinforcementsusingcentrifugalmixer andprintingusing3mlsyringe Strengthof172MPaandstiffnessfrom 2GPato5.5GPa.15wt%silicananoparticleswereaddedtotheresinsuch thatitbehavesasathixotropic,non-Newtonianfluid whichimprovedflowofthefibres.

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inter-bead voids and fibre-pull out. Tekinalp et al. [25] found a re- duction of inter-bead voids with the addition offibres, which was attributed to a decrease in die swell and increase in thermal conductivity, which helps the surrounding beads to soften and improve polymer sintering. Smaller voids, however, were found around thefibres which increased with higher fibre contents. This was attributed to a weak fibre-matrix interface and partially independent movement of thefibres and matrix during extrusion. Lastly, Zhang et al. [34] found an increase in porosity with the addition offibres to ABSfilament which shows the effect of reinforcement on the porosity is not fully understood.

Bellini [19] found numerically that a high thermal conductivity of thefilled material (roughly a factor of 7 higher than unfilled material) improves heat transfer from the liquefier to the printed material, improving overall flow. The addition of fillers to the printingfilament reduces die swell, as reported in three different studies [19,26,28]. The addition of fibres may be used to alter the thermal energy transfer between printed beads during deposition and theflow and the swelling behaviour of the material when leaving the nozzle.

Another promising, albeit less common, approach to 3D printing composites is to use continuousfibre reinforcedfilament. MarkForged has developed a printer which deposits continuousfibres (carbon, glass or Kevlar) in a Nylon matrix. The manufacture reports strength and stiffness of printed parts with carbonfibres of 700 MPa and 50 GPa respectively [2]. A∼0.4 mm diameter continuousfibre/Nylonfilament is fed through a nozzle and, after it is initially anchored to the printing bed, dragged along a custom path. As it is printed, thefibre reinforced filament is transformed from an initially round cross-section to a rectangular one, with a significant amount of compression andflattening

occurring to improve in-ﬁll and inter-laminar bonding. This process, and its limitations are discussed further in the experimental section in more detail.

Yang et al. [36] developed a novel composite extrusion head, where dry carbonﬁbre is fed through a melt pool of ABS. This increased in- plane mechanical properties by a factor of 2–5, but a limiting factor was the interlaminar shear properties of the printed part. Matsuzaki et al.

[26] printed continuous fibres (straight carbon fibres or twisted jute fibre yarns) by feeding them through a nozzle simultaneously with a thermoplasticfilament (PLA) which acts as a matrix. They reported a strength and stiffness of 195 MPa and 10.5 GPa respectively which may be attributed to a low Vfof 6.6%. This technique also showed a non- uniformfibre distribution as thefibres were not pre-impregnated in the matrix.

From the review of the composite 3D printing technology presented above, two main printing methods approaches can be identified: the printing of short (0.1 mm) fibres with traditional material extrusion based printing methods and continuousfibre printing with a custom printing head and technique. Despite multiple studies available on both methods, there does not seem to be a clear consensus how these two methods compare in terms of printing versatility, print quality and mechanical properties. To better understand the two methods and how they compare, both will now be evaluated in terms of mechanical properties and printing characteristics before drawingfinal conclusions on how FFF may be used to manufacture cost-effective, high quality parts with good mechanical performance.

3. Experimental methodology

The part quality and mechanical performance of 3D printed composite parts manufactured using two different printing methods are investigated here. Continuous carbonfibre/Nylon 3D printed parts are made using the Mark Forged MarkOne printer and discontinuous carbon‘microfibre’reinforced Nylon parts are made using a standard desktop 3D printer. Various experiments are performed to quantify key mechanical properties, including the most detailed set of mechanical tests on the MarkOne printed parts reported to date, and optical microscopy is used to examine the quality of the parts.

3.1. MarkOne continuousﬁbre printer characterization

The MarkOne printer is a proprietary 3D printer which can deposit a filament made of continuousfibres embedded in a Nylon matrix. The printer has two printing nozzles as shown inFig. 7, one to deposit pure Nylonfilament, and one forfibre reinforced Nylonfilament. The unreinforced Nylon nozzle is crucial for the overall integrity and quality of the prints, as thefibrefilament cannot be used for the outer layers of the parts (top, bottom, sides), and for more complex shapes and thin Fig. 6.Different categories of porosity in 3D printed carbonfibre composites,

(1) gas bubbles (2) interbead voids and (3)ﬁbre pull-out [35].

Fig. 7.Overview of the MarkOne Printer with the dual nozzle system to print Nylonfilament andfibrefilament.

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features there often are large regions in which thefibrefilament is not able tofill, which instead arefilled with the unreinforced Nylon. The Nylon filament and Nylon/fibre filament are fed through Teflon Bowden tubes which run between the drive motors and the nozzles, as highlighted inFig. 7.

To print an object, a proprietary slicing software must be used. This software is“closed source”and does not allow for user adjustment of key printing parameters such as temperature, nozzle movement or extrusion speed. This limits the printing capabilities as the printing set- tings cannot be fully customized. For the deposition of carbonfibres, only a circumferentialfill pattern is possible whichfills the shape from the outside inward in a spiralling motion. This means thefibre is always orientated along the outer perimeter of the part.

A 3D object is sliced into layers with a layer height of 0.125 mm for carbonfibre reinforced layers. The bottom and top layers are always printed with the 100% triangular fill Nylon filament, as well as the outer periphery for each layer. This is presumably done to avoid ex- posedfibres on the outer surface and to take advantage of the higher quality surface finish and accuracy available from the unreinforced Nylon. While this feature exists for sensible reasons, it has the negative effect of lowering the overallfibre volume fraction for the part - and thus the maximum achievable mechanical properties. Printing is done at 260 °C with at an estimated speed of 6.90 cm³/hr for the Nylon layers and 2.39 cm³/hr for the carbonfibre layers.

A signiﬁcant downside of Nylon is that it is sensitive to water absorption, which plasticizes the matrix and can lead to a decrease in strength of up to 33% [37].

To provide a reliable and useful benchmark of the MarkOne printer, the most extensive suite of mechanical tests and printing trials reported to date were performed. The tensile,flexural and shear response of the printed material have been measured. To get around the limitations in fibre orientation caused by the circumferentialfill pattern, the tensile specimens were printed in an“oval racetrack”shape to allow for two unidirectional 0° specimens to be extracted from each print, as shown in Fig. 8. The dimensions of the tensile specimens were 250 mm × 15 mm × 1 mm, sized in accordance to ASTM standard D3039 [38], where the bottom and top layers of 0.125 mm thick were 100% triangularfill Nylon as discussed above. The volume fraction of these specimens was Vf≈27%, estimated using optical microscopy.

Glassﬁbre tabs with a length of 25 mm were bonded to the specimen using an epoxy adhesive and the tensile test was carried out at constant displacement rate of 2 mm/min in a servo-hydraulic machine. Strain measurements were obtained from a video extensometer (IMETRUM, UK) over a gauge length of 100 mm, and the load was obtained from a 25 kN load cell (Instron).

A three-point bendfixture was used to obtain theflexural properties of the printed composite material with a support rod radius of 4 mm and a support length of 128 mm according to the ASTM D7264 standard [39]. Theflexural specimens were manufactured in a similar approach as the tensile specimens, with outer dimensions of 160 mm × 11 mm × 4 mm and allfibres orientated in the 0° orientation. A constant displacement rate of 1 mm/min was used and the force and displacement were directly measured from the machine with a 1 kN load cell.

To obtain the shear properties, the methodology proposed by Sun and Chung [40] was used for uniaxial off-axis testing with oblique end- tabs, as a ± 45° specimen could not be printed. The oblique end tabs help create a uniform state of stress from which the shear properties can be obtained. The required oblique angle is a function of the chosen off- axis angle of thefibres and the properties of the composite, which were estimated from the results of the previous tests to be E11= 50 GPa, E22= 0.38 GPa, G12= 3.9 GPa andν12= 0.3. A 1 mm thick plate was printed to extract the shear samples. The sample dimensions were 200 × 10 × 1 mm, with thefibres orientated at 13°. The required angle for the oblique end-tabs for a uniform state of stress was 21° [40], as shown in Fig. 8b. The specimen was tested using an electrical-mechanical tensile machine with a 10 kN load cell and strain measurements were obtained from a 5 M P LaVision DIC system.

Lastly, to assess the printing performance of the MarkOne printer and the quality of the continuousﬁbres deposited, benchmark parts were printed in the form of a 40 × 40 mm square, a 30°–60°–90° tri- angle (85 × 50 mm) and a circle with a radius of 40 mm. Defects in printed parts due to the diﬀerent geometry conditions were investigated using inspection and optical microscopy on the printed samples.

3.2. Short carbonﬁbre nylonﬁlament characterization

Shortfibre reinforced Nylon parts were printed using a Lulzbot TAZ 6 printer and a Nylon filament which was reinforced with chopped carbonfibres. The material was acquired from Fiberforce Italy (under the brand name Nylforce) and has 6 wt% carbonfibres added to the 3.00 mm diameterfilament [41]. In comparison to the Mark One, an open-source printer allows far more control of the material deposition strategy, such as printing tracks, extrusion rate and printing temperature. To characterize this printing technique and the material, tensile, flexural and shear specimens were printed to determine the mechanical properties. Similarly, optical microscopy was used to investigate the quality of these specimens.

Tensile specimens were printed in dog-bone shapes according to the ASTM D638 [42], using a 0.4 mm nozzle diameter, 0.2 mm layer height and a printing temperature of 260 °C as recommended by thefilament manufacturer. A 0°fill pattern was used such that the gauge section of the dog-bone specimens consists of tracks aligned in the 0° direction as shown inFig. 9. Theflexural specimens were printed as rectangles with dimensions 168 × 13 × 4 mm to match the ASTM D7264 standard for three point bending [39] with a support length of 128 mm. Shear samples were printed based on the ASTM D3518 standard for in-plane shear of composites, with the geometry of a dogbone and a [ ± 45]8s

layup. The x- and y- strain components were measured at the gauge section using a video extensometer to obtain the shear modulus and strength. To assess the printing performance with the carbon ﬁbre/

Nylonﬁlament, similar benchmark parts to the MarkOne benchmark parts were printed to investigate the corner radii and quality.

4. Results

4.1. MarkOne continuousﬁbre printer characterization

The results of the tensile tests on the specimens printed by the MarkOne printer are shown inFig. 10a. Four composite samples were tested which show an average strength and stiﬀness of 986 MPa Fig. 8.Print schematic of (a) unidirectional tensile andﬂexural specimens and

specimen extraction and (b) shear specimens, showing carbonﬁbre path.

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( ± 8.3%) and 62.5 GPa ( ± 4.9%) respectively, which are higher than reported by MarkForged [2]. During the test, characteristic high fre- quencyfibre fracture sounds were heard at a low stress of 200 MPa, after which no fracture was heard before failure. InFig. 13a, a slight stiffening effect can be seen where the slope increases at higher strains.

This may indicate early fibre fracture of possibly wrinkledfibres, followed by relaxation of most thefibres which leads to better alignment at higher load levels.

The results of theflexural tests are shown inFig. 10b. Theflexural modulus and strength of the carbon specimens are 41.6 GPa ( ± 4.3%) and 485 MPa ( ± 1.0%) respectively. Theflexural strength is lower than the tensile strength which indicates there may be issues with the quality of the specimen, as a higher flexural strength is expected for high qualityfibre composites [43]. A compressive failure was found for these specimens relating to a poorfibre/matrix interface and/or a high void content as failure initiators.

The shear response of the off-axis unidirectional specimens is shown in Fig. 10c. The modulus has been determined from the initial linear part of the curve, which was found to be 2.26 GPa ( ± 4.9%) and is lower than the predicted value of 3.9 GPa. The predicted value was used to determine the oblique end-test tab angle to obtain a uniform stress state. The difference may influence the results as a non-uniform state of stress occurs which can lead to premature failure. The maximum shear stress was 31.16 MPa ( ± 15.8%) where it must be noted that specimen 2 failed near the tab, which may explain its lower shear stress.

The printing quality of the MarkOne can be seen inFig. 11with the printing of various generic shapes. The MarkOne printer prints the carbon fibres as a continuous path which spirals from the outside contour to the inside. For more complex geometries, this causes large fibreless areas as shown in the triangular part. Thesefibreless areas can

be up to 2.5 mm × 1 mm, which are recognized by the printing software and are partiallyfilled with pure Nylon, but this leads to a local weakness in the part. For a simpler shape, such as a square, a similar effect is present in the corner regions on a smaller scale (1.5 mm × 0.3 mm) but the areas are notfilled with Nylon here which leads to voids. For the circular shape, thefibres neatly follow the outside contour with some small gaps with a width of 0.05 mm.

Optical microscopy was performed to further asses the printing quality of the MarkOne printer.Fig. 12shows the unprintedfibre reinforcedfilament, which has a nominal diameter of 400μm. The carbon fibres seem to be localised in three bands and some voids can be seen as dark spots. The Vfin thefilament was estimated using ImageJ software with a greyscale threshold and was found to be 20%.

Fig. 13is a micrograph of the cross section of one of theflexural test specimens showing the multiple stacked layers through the thickness of the part. Within each layer distinct regions of Nylon,fibre, and void can be seen. Thefibre volume content is estimated to be 27% over the cross section, which is higher then in the filament which is attributed to possible non-uniformity of thefilament. Additional voids may be cre- ated during the printing process as thefilament is non-uniform, forming airgaps between tracks. A void content of 7–11% was estimated from the micrograph using ImageJ and a greyscale threshold. Moreover, a non-even distribution offibres can also be seen in the printed tracks.

4.2. Carbonﬁbre nylonﬁlament characterization

The result of the tensile tests andflexural tests on the short carbon fibre Nylonfilaments are shown inFig. 14. The average tensile strength and stiffness are 33.5 MPa ( ± 2.7%) and 1.85 GPa ( ± 6.1%) respectively. The properties are lower than the bulk properties of Nylon [44], showing that the fibres do not reach their ultimate strengths. The flexural strength and stiffness were found to be 55.3 MPa ( ± 3.4%) and 3.0 GPa ( ± 4.1%) respectively. For the reinforced carbonfibre Nylon filament, theflexural strength is higher than the tensile strength, which is expected for a high quality composite part [43]. The shear results are shown inFig. 14c. The shear strength and modulus were found to be 19.02 MPa and 0.31 GPa, respectively. One comment here is that this test method assumed orthotropic ± 45° layers, while clearly the short carbonfibre Nylon part behaved more like a plastic part, with a large amount of plastic deformation.

Fig. 15shows the micrographs of parts printed with short carbon ﬁbre reinforced Nylon. The printed part (Fig. 15a) shows characteristic triangular voids between the printed tracks from the FFF process. Using ImageJ software and a greyscale threshold, the total void content was estimated at 1.1%, with mainly triangular voids from the printing process).

Fig. 15b shows a 90° corner region of a printed part, showing the change in orientation of thefibres. It also shows that considerablefibre pull-out has occurred during cutting and polishing of the sample–indicating a lowfibre-matrix adhesion. Some voids are present, but the gap between the printed tracks is much smaller compared to the con- tinuousfibrefilament (Fig. 11).

Fig. 9.Dog-bone, shear andﬂexural specimen printingﬁll patterns and specimen dimensions.

Fig. 10.Tensile,ﬂexural and shear test results of MarkOne continuousﬁbre printed specimens.

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4.3. Comparison of continuousﬁbre printing and shortﬁbre printing

Table 2 shows a comparison of the printing methods, where the mechanical properties have been normalised by ratio to a Vf of 15%.

The tensile properties of the continuousfibre 3D printed samples were roughly an order of magnitude larger, which can be attributed to the fact that the shortfibres did not reach their full strength. Theflexural properties of the continuous fibre parts were lower than the tensile properties, indicating quality issues [43]. Theflexural properties of the shortfibre 3D printed parts were higher than its tensile properties, but still a factor 3 lower compared to the continuousfibre printing method.

The shear properties of both printing methods are closer together, with

the shortfibre parts showing a relatively high shear strength. Together with the lower porosity, it shows that the shortfibre printing method produces a higher quality part than the continuous fibre printing method.

5. Discussion

Fused filament fabrication has been investigated as a low-cost manufacturing method for fibre reinforced composites materials. An important aspect of composite materials is the consolidation of thefibres into the matrix. Traditional automated composite manufacturing techniques such as automated tape placement (ATP) use additional consolidation rollers and an autoclave process to improve thefinal part quality [45]. Compared to ATP machines, 3D printers are simple in design and use but lack the ability to apply additional pressure and heat to the part.

From the current body of work on composite 3D printing it must be concluded that the quality of a 3D printed part is still low compared to classical aerospace grade composite materials, as literature and this study showed void contents in the order of 10% are not uncommon for 3D printed parts. Multiple studies report on an increase in mechanical properties from unreinforced to reinforcedﬁlament, but to be used as a structural material the absolute strength and stiﬀness must increase as well as the consistency and quality of manufactured parts.

The coupled thermo-ﬂuid-mechanics of the material extrusion 3D printing process has been carefully analysed to identify a method forward to improve the quality of 3D printed composite parts. The basic extrusion process is well documented, but the literature lacks a Fig. 11.Benchmark prints for MarkOne printer with detail of corner radii.

Fig. 12.Cross section of the MarkForged carbonﬁbreﬁlament.

Fig. 13.Cross section of printed MarkForged part showing structure from 3D printing tracks and distribution of voids.

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coherent understanding of how different printing parameters affect final part quality.

The ideal properties for a 3D printingfilament are summed up in Table 3, categorized into processing and performance properties. The flow properties of the polymer are important for the polymer sintering process, which ideally consists of a low melt viscosity and high surface energy. The thermal properties of a 3D printing filament dictate the thermal energy history of the printed tracks, where it is important that the extruded material and the surrounding material reach a high enough temperature and maintain that temperature for long enough to enable bonding between adjacent tracks. A higher heat capacity means the material needs more heat input to increase its temperature but can also store more heat once it leaves the liquefier, while a high conductivity is required to transfer the heat to the surrounding material.

After sintering, the material cools down which may induce residual stresses, so ideally the material has a low melt/glass transition temperature, which is a conflicting requirement with a high operating temperature. The desired mechanical properties of the polymer are a high stiffness and strength, and potentially a good interfacial strength when reinforcingfibres are used.

Studies indicated that the addition of fibres to the filament may improve the heat transfer between printed tracks, leading to a better sintering process and reducing void content. However, fibres also increase the viscosity of the melt which has a negative effect on the sintering process as some studies showed a higher void content with fibres. A larger, more extensive study is proposed wherein the effect of differentfillers, printing temperatures and printing strategies are investigated in order to reduce the void content and obtain higher quality parts.

Another important aspect for the printing of composite materials is the use of short versus continuousﬁbres. Currently, a limited number of commercial products are available for both, but rigorous, comparative material testing with detailed consideration of defects (through optical microscopy) has until now not been available in the public domain. The test results of the MarkOne continuous ﬁbre printed parts presented here indicate good mechanical properties which are an order of magnitude higher than typical FFF printed materials, although still

signiﬁcantly lower than unidirectional composites made with traditional manufacturing methods (strength/stiﬀness of 1500 MPa/

135 GPa). Placement of continuous fibre filament is limited by a number of geometric and processing constraints, such as a minimal deposition length and minimal corner radii. Shortfibre printing allows for considerably more freedom in where and how the reinforcement is placed, resulting in easier processing of the material and lower void content. The mechanical properties are relatively low as the matrix or fibre-matrix interface fails before thefibres.

The results agree with the known trade-offbetween processing and performance as shown inFig. 16. To further optimizefibre reinforced 3D printing materials, highly aligned shortfibres withfibres above the criticalfibre length may provide a good trade-offbetween processing and performance. For carbonfibre in a Nylon matrix, the criticalfibre length is roughly 0.5 mm [46]. Investigations have shown that aligned shortfibre / epoxy composites (Vf= 55%) can obtain a strength and stiffness of 1500 MPa and 115 GPa with the HiPerDiF method [48,49].

Increasing thefibre length therefore may be a way forward to an 3D printingfilament with the advantages of rapid prototyping and compete with continuousfibre mechanical performance.

Fig. 14.(a) Tensile, (b)ﬂexural and (c) shear test results of carbon microﬁbre reinforced Nylon.

Fig. 15.Microstructure of 3D printed shortﬁbre Nylon showing (a) cross section and (b) top view of corner.

Table 2

Comparison printing methods with normalised mechanical properties toﬁbre volume content of 15%.

Shortﬁbre printing method

Continuousﬁbre printing method

Brand name Nylforce MarkForged

Fibre volume content 6% 27%

Porosity 1.1% 9%

Measured Normalised Measured Normalised

Tensile modulus [GPa] 1.85 4.6 62.5 46.9

Tensile strength [MPa] 33.5 83.8 968 726.0

Flexural modulus [GPa] 3 7.5 41.6 31.2

Flexural strength [MPa] 55.3 138.3 485 363.8

Shear modulus [GPa] 0.31 0.8 2.26 1.7

Shear strength [MPa] 19 47.5 31.16 23.4

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6. Conclusions

In this work the state of the art of 3D printed composite parts has been presented, and the performance of two of the most advanced so- lutions currently available have been benchmarked with mechanical testing and optical microscopy. Printing of shortfibre (∼0.1 mm) reinforced Nylonfilament was performed using a standard open-source FFF printer and a MarkOne 3D printer was used to print continuous carbon fibre / Nylon composite specimens. The tensile strength and stiffness of the continuousfibre printed parts were 986 MPa and 64 GPa respectively, which is more than an order of magnitude higher than the short fibre reinforced Nylon printed parts (33 MPa and 1.9 GPa). A disadvantage of the continuousfibre printer, however, is limited control over the placement of thefibre and the creation of voids when printing more complex shapes. To overcome these disadvantages, a thermoplastic filament reinforced with shortfibres above the critical fibre length is proposed. This would yield mechanical properties similar to continuous fibre prints while maintaining the better processing qualities of short fibre reinforcedfilament. This may enable new applications for high performance 3D printed parts suitable for medical, aerospace, sport and rapid prototyping applications.

Acknowledgements

This work was supported by the Engineering and Physical Sciences Research Council through the ACCIS Doctoral Training Centre [grant number EP/G036772/1]. Data access statement All underlying data supporting the conclusions are provided in full within this paper.

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